1. Field of the Invention
The present invention relates generally to micro-fluid ejection devices and, more particularly, to a micro-fluid ejection device and a method for assembling the micro-fluid ejection device by wafer-to-wafer bonding.
2. Description of the Related Art
Micro-fluid ejection heads or devices are broadly useful for ejecting a variety of fluids including inks, cooling fluids, pharmaceuticals, lubricants and the like. One widely-practiced use of a micro-fluid ejection device is as an inkjet printhead in an inkjet printer. The primary components of the inkjet printhead are an actuator chip, a nozzle plate attached to or integrated with the actuator chip, and a flexible circuit for electrically connecting the actuator chip to the printer during use. The actuator chip is typically made of a silicon substrate and contains various layers built up into stack form at a front surface of the silicon substrate using well-known microelectronic fabrication techniques.
Fluid ejection actuators formed on the substrate surface of the actuator chip may be thermal actuators or piezoelectric actuators. For thermal actuators, typically scores of microscopic resistive heater elements are defined in a resistive layer, each resistive heater element being aligned with and corresponding to one of scores of microscopic nozzle holes in the nozzle plate for heating and ejecting a fluid, such as ink, from the nozzle hole toward a desired substrate or target, which in the case of an inkjet printhead is usually print media. It can be readily appreciated that slight misalignment of the nozzle holes with the heater elements can adversely affect the quality of the print made on the print media.
The realization of ultimate inkjet print quality is influenced by several factors, of which one important driving force is the precise placement of ink drops on the print media upon expulsion from the nozzle holes of the inkjet printhead nozzle plate. Currently, the most prevalent techniques for nozzle plate formation are the so-called “pick and place” of polymer nozzle plates with pre-formed nozzle holes, and the photoimagable polymers in which the nozzle holes are formed once the polymer is applied to the chip. These photoimagable polymers may be spun on or laminated. These technologies are limited by shortcomings in accuracy and precision with which the nozzle holes can be located over the heater elements on the chip, thereby adversely affecting print quality. The “pick and place” method of nozzle plate formation is severely limited by the alignment tolerances associated with the placement of the nozzle plate and also by the shortcoming in accuracy and precision of the laser ablation process typically used to form the nozzle holes. The photoimagable processes, although an improvement, are still limited by the materials mismatch between the polymer nozzle plate and silicon wafer leading to differential expansion/contraction with thermal cycling and also by the inherent instability and flexibility of polymer materials. For example, problems such as sagging of the nozzle material over the ink via and distortion of features due to internal stresses are easy to imagine. Additionally, since the nozzle holes are formed by wet chemical development of a photo-exposed area, the nozzle hole size and shape can be difficult to control. All of these factors can degrade print quality by affecting the placement and/or geometry of the nozzle holes.
A third technique for nozzle plate formation is to deposit a thin film over a sacrificial polymer material, pattern the film to form nozzle holes, and subsequently remove the polymer in order to form the ejector chamber. This method for forming a nozzle hole has the benefit of using a ceramic or metallic film as the nozzle layer, thereby improving compatibility with the substrate and providing improved rigidity and thermal stability. However, this method requires depositing a film over the top of a polymer and thus represents a trade-off between a polymer capable of withstanding thin film deposition temperatures and a thin film that can be deposited to sufficient thickness and with desired properties at a moderate temperature to prevent polymer decomposition. Additionally, this process typically results in a very irregular and undulating surface, which may present maintenance concerns.
Thus, there continues to be a need for an innovation that will improve the components of the inkjet printhead and their assembly to one another in order to improve or enhance print quality.